The present invention relates to the examination of a radiograph of an object obtained with the aid of ionizing radiation from a source, said object having in the extent of its volume areas of variable transparency to said ionizing radiation.
In known processes of this type, the intensity of the ionizing radiation which has passed through each volume element of the object to be radiographed is generally detected by an ionization chamber in which the radiation causes electrical charges of opposite signs, whose location and intensity characterize the absorbing properties of the investigated object and constitute a latent electrical image of the thus examined object.
Thus, this latent image is formed from a certain number of electrical charges distributed in the detection volume of an ionization chamber, which is generally filled with an ionizable gas and has the property of being rapidly removed because the thus formed charges (electrons and ions), on appearance, immediately move towards the collecting electrodes of the chamber constituting the anode and cathode thereof. The problem of examining the thus obtained latent image is consequently difficult to solve, particularly as in the presently known processes it is generally necessary to work with the aid of very short ionizing radiation flashes (X, gamma or neutron rays, using a suitable converter), which correspondingly reduces the intensity of the charges produced in the volume of the chamber.
The charges created in this way by ionization produce, by influence on each electrode, charges of opposite signs and equal values, which, transformed into a detectable electrical current, immediately give an overall value of the sum of the instantaneous charges constituting the latent image. However, this unfortunately does not make it possible to achieve the desired cartography of the latent image along the OX and OY coordinates, which are respectively parallel and perpendicular to the electrical field of the chamber.
To obviate this difficulty, consideration has been given to producing ionization chambers in the form of mosaics of several juxtaposed elementary chambers, thereby making it possible to mark along the two preceding coordinates, the distribution of the space charges induced by ionization from the ionizing beam. However, the existence of partitions between the different adjacent chambers constitutes an important limitation to the accuracy of detection and also the spatial resolution of such an apparatus is obviously limited by the dimensions of the elementary volume of each detection chamber.
Consideration has also been given to the use of processes for collecting the electrical charges formed on insulating plates, so that a bidimensional latent image is obtained in this way. It is then possible to read this image by appropriate electrical devices or develope it in the manner of a photographic plate by optical reading such as that used in xerography.
In general terms, the prior art referred to hereinbefore leads to major disadvantages, which make it virtually impossible to examine the radiograph of an irradiated object with the aid of an ionization radiation source, by using as the detector an ionization chamber.
Thus, the so-called mosaic ionization chamber means have a spatial resolution, which is limited by the size of the juxtaposed elementary cells and which have a complex and therefore costly construction. The means for collecting charges on insulating plates necessarily require special devices for measuring and locating these charges on the surface and in the present state of the art these processes are extremely slow.